Optical resonance cell with means for regulating internal vapor pressure

ABSTRACT

An optical resonance cell containing an alkali vapor to be optically pumped, the transparent walls of which are coated with a layer of paraffin while a bulb containing the reserve of alkali metal is connected to the cell in such a manner as to decelerate the alkali vapor flow between the bulb and the cell.

United States Patent Brun 1451 July 4,1972

[54] OPTICAL RESONANCE CELL WITH MEANS FOR REGULATING INTERNAL VAPOR PRESSURE [72] Inventor:

[73] Assignee: CSF-Compagnie Generale de Telegraphite Sans F1] 22 Filed: March 16,1971

21 App1.No.:.124,974

Henri Brun, Paris, France Related US. Application Data [63] Continuation of Ser. No. 796,236, Feb. 3, 1969, abandoned.

[30] Foreign Application Priority Data Feb. 2, 1968 France ..68138532 [52] US. Cl ..313/174, 324/05 F [51] Int. Cl. ..1I0lj 61/24 [58] Field of Search ..313/174, 181; 331/945;

[56] References Cited UNITED STATES PATENTS 3,353,037 11/1967 Jester et a1 ..'.....313/174 X 2,030,404 2/1936 Ryde et al.... ..315/108 2,081,247 5/1937 Mulder ....315/108 X 2,280,618 4/1942 Besson..... ....313/174 X 3,242,423 3/1966 Malnar ..324/.5 F 3,331,977 7/1967 Wainio .313/220 X 3,486,058 12/1969 Hernquist.... ..33l/94.5 3,084,257 4/1963 Goddard ..313/174 X FOREIGN PATENTS OR APPLICATIONS 1,036,002 7/ 1966 Great Britain ..324/.5 F

Primary Examiner-Roy Lake Assistant Examiner-Palmer C. Demeo [57] ABSTRACT An optical resonance cell containing an alkali vapor to be optically pumped, the transparent walls of which are coated with a layer of paraffin while a bulb containing the reserve of alkali metal is connected to the cell in such a manner as to decelerate the alkali vapor flow between the bulb and the cell.

7 Claims, 2 Drawing Figures OPTICAL RESONANCE CELL WITH MEANS FOR REGULATING INTERNAL VAPOR PRESSURE This application is a continuation of Ser. No. 796,236, filed Feb. 3, 1969, now abandoned.

The present invention relates to optical resonance cells which can be used, for example, in atomic clocks or optically pumped magnetometers.

At the present time, cells are available which are capable of operation only at very low optimal temperatures. In other words, these cells can operate only with an optimum density of a saturating alkaline metal vapor; if this density is too low in the cell, too few atoms are involved in the phenomenon of optical pumping and resonance, and the resonance signal disappears. Conversely, if the vapor density is too high, there is too high a degree of absorption of the pumping light wave, due to the fact that, because of the relaxation phenomenon, the atoms of a vapor are never all oriented, and the output signal reaching the optical detector is too weak.

The optimum temperature of operation of a resonance cell is for example 35 C in the case of caesium vapor and 45 C in the case of rubidium vapor. The regulation of the temperature of such cells, at ambient temperatures which may be higher than these temperature levels, requires the use of thermoelectric devices (such as frigatrons) which take a relatively large amount of power.

One solution has been considered with a view to avoiding the need for the use of such devices. A specific compound of carbon and alkaline metal is used which enables a suitable alkali vapor pressure to be achieved by the heating of said compound to a temperature in the order of 250 C or more. However, this approach has the drawback that part of the resonance cell has to be heated to 250 C, and this may be detrimental in so far as the stability of the system which comprises the cell, is concerned, due to convection currents which develop inside the cell.

It is an object of the invention to overcome these drawbacks of the earlier proposed systems and to provide a resonance cell the control of whose temperature is a straight-forward matter in all normal conditions of operation, this without excessive power consumption, the desired result being achieved by operation of the system at a relatively high temperature although the optimum vapor density is maintained.

According to the invention, a specific property of absorption of atoms of the alkali vapor is exploited. This property is exhibited for example by a layer of substance, such as paraffin, deposited upon the walls of the cell, which layer has already been used in certain known cells with a purpose similar to that for which a filler gas is used, that is to say in order to prevent disorientation of the vapor atoms as a consequence of direct collision with the cell walls (the mean free trajectory of the vapor atoms being greater than the cell dimensions). This property is again exhibited by the glass of the wall itself, if a filler gas is being used.

According to the invention, there is provided an optical resonance cell comprising an envelope having transparent walls; a bulb containing an alkali metal and communicating with said envelope; first means for decelerating the alkali vapor flow between said bulb and said walls; and second means for absorbing on said walls a predetermined number of alkali atoms of said vapor flow.

For a better understanding of the invention and to show how the same may be carried into effect, reference will be made to the drawings accompanying the ensuing description and in which:

FIG. 1 illustrates an optical resonance cell in accordance with the invention; and

FIG. 2 is an explanatory graph.

The cell according to the invention, illustrated in FIG. 1, comprises an envelope 1 of any desired shape, the walls of which are covered with a fine layer or film 2 of a substance which, on the one hand, prevents any disorientation of the vapor atoms as a consequence of collision with the walls, and, on the other hand, has a certain capacity for absorption of said atoms. This layer will be, for example, of paraffin.

The cell likewise comprises an exhaust stem 3 and a bulb communicating with the cell and containing alkaline metal 5, the vapor of which is to be optically pumped. In accordance with the invention, the bulb 4 is connected to the envelope 1 through a tube 6 having a neck 7 of very small diameter.

The layer 2 should be sufficiently thin to enable the envelope to retain its transparency to the pumping light. The operation of the cell is based upon a specific property of the paraffin layer 2, namely that it absorbs per second, a certain number of atoms of the alkaline vapor, which number is a function of the temperature and of the number of atoms in the envelope.

in known cells, in which a parafiin layer is employed in order to prevent disorientation of the vapor atoms by collision with the cell walls, this absorption has no effect from the point of view of the vapor pressure within the envelope, since the reserve of alkaline metal at all times compensates the atoms lost by absorption by the parafiin and necessary to establish the saturation vapor pressure at the envelope temperature, which temperature will have to keepan optimum value of, for example, C in the case of caesium and 45 C in the case of rubidiumQln contradistinction to that, in the case of the invention the flow of gas atoms supplied by the alkaline metal reserve contained in the bulb 4 is limited, is order to enable the cell to operate at a higher temperature, while at the same time maintaining the optimum vapor pressure, for example P i.e. the pressure corresponding to the saturation pressure at 45 in the case of rubidium. The operation of the cell in accordance with the invention can be explained in the following manner:

Let it be assumed that it is desired to operate the cell at a temperature 0, the envelope 1 and the bulb 4 thus being brought to this temperature while maintaining within the envelope the optimum vapor pressure P,,,, assuming that the alkaline metal is rubidium. In the bulb, the vapor pressure is P= above P if 6 is higher than 45 C. At the temperature 0 the paraffin layer 2 absorbs n atoms per second for a number n of rubidium atoms in the envelope corresponding to a vapor pressure of P In addition, because of the pressure difference between the bulb and the envelope, there is supplied through the neck 7, to the envelope, a stream of n, atoms per second. Thus, equilibrium, and therefore the optimum pressure P is maintained inside the cell, even if the latter is brought to a temperature 0 if n,, n,.

By way of example, considering a neck 7 of radius r, the number n of rubidium atoms, supplied per second to the envelope, is given by:

-1 A p VR.T.M.

bulb 7 and the envelope 1, N is the Avogadro fa ctor, R the gas constant, T the absolute temperature 0 273.15 of the vapor, and M the atomic mass of the vapor.

In the M K S A system this gives n, 1.5 X10 FAp.

Also, it is possible to determine experimentally the absorption characteristic of the parafiin layer 2 as a function of the temperature 0 of the cell. FIG. 2, shows by way of example curves which respectively illustrate the variations in 11, and n, for a given paraffin layer and a given diameter of the neck 7, the rubidium vapor pressure in the envelope being Pm. The transition point at around 77 C, for n,,, corresponds to the melting point of the particular paraffin. There are three points for which n n,

Two stable points, one at around 45 and the other at around 80", and an unstable point at 77.

Preferably, an operating temperature no higher than the melting temperature of the parafiin will be used, because absorption then becomes very fast and this means that a substantial alkaline metal reserve is then required. Accordingly, the

curve n is modified by reducing the diameter of theneck 7, in order to produce curves for n and n, which are substantially coincidental between 45 and 77 C.

In this way, a cell is obtained which produces an optimum signal in a continuous fashion, throughout the whole temperature range between 45 and 77 C.

The manner in which the cell in accordance with the invention starts operating, is easily explained. At ambient temperature, the envelope will tend to empty itself of vapor. When the envelope is brought to the predetermined operating temperature, there is initially a low number n of atoms in the envelope. However, the absorption factor of the paraffin is proportional to the number of atoms present in the envelope:

. n,, A n where A is a constant.

n, is therefore low at the start and n, is much higher because of the pressure difference between the bulb and the envelope. Accordingly, the envelope starts to fill up. As filling progresses, n rises and n, reduces, until an equilibrium (n, n is reached, where the number of atoms in the envelope remains the same and this condition maintains in the envelope the desired optimum vapor pressure.

In one embodiment, operation at 70 C was made possible using a 3/ 100 mm diameter for the neck 7.

Similar results can also be obtained with cells containing a filler gas, the glass walls of which do not carry any paraffin layer. In cells of this kind, the alkaline substance reacts chemically with the glass and the walls of the envelope therefore absorb a certain number of atoms and play the same part as the paraffin layer in the example hereinbefore described. On the other hand, there is then no point in decelerating the gas flow between the-wall of the envelope and the alkaline reservoir, using a small-diameter orifice or neck.

The filler gas is by itself adequate to produce the desired decelerating effect, due to the diffusion of the alkaline vapor through the gas. In order to regulate the temperature of operation and achieve the desired equilibrium condition, all that is necessary is to vary the length and width of the neck 7 connecting the bulb 4 with the envelope 1, the gas flow being highly sensitive to these parameters.

The cells according to the invention have, among others, further advantages:

A higher operating temperature than known cells, although the optimum vapor pressure is nevertheless maintained within the envelope.

This means there is a facility for control of the cell temperature, without having to resort to excessive power consumption.

A lower degree of necessary precision in said control, since the optimum vapor pressure can be achieved in the envelope not merely for a single temperature but throughout a continuous range of temperature.

Operation at a high temperature without any necessity for heating up one part of the cell more than any other, as was required in the case of cells using a compound of carbon and alkaline metal, thus meaning improved stability in devices employing the cell in accordance with the present invention.

Of course, the invention is inno, way limited to the examples described hereinbefore. In particular, the alkaline metal used need not necessarily be rubidium.

In addition, it goes without saying that the arrangement for decelerating the gas flow between the bulb 4 and the envelope 1 is not necessarily a small-diameter orifice or neck (in the case of envelope lined with an absorbent layer), and other arrangements having the same function, may be placed at 7, for example a porous wall or a capillary tube, may be used, the gas flow then depending not only upon the diameter of the capillary tube but also upon its length.

I claim:

1. An optical resonance cell adapted to substantially maintain an optimum alkali metal vapor pressure therein over a range of temperatures, said cell comprising;

an envelope having transparent walls,

a reservoir containing an alkali metal and connected to said 7 envelope for supplying vaporized alkali atoms thereto, first means disposed between said reservoir and said envelope for limiting said supply of alkali atoms to a first rate which is a first predetermined function of temperature, and second means within said envelope for absorbing said vaporized alkali atoms from said envelope at a second rate which is a second predetermined function of temperature,

said first and second means being constructed relative to each other and cooperating with each other to cause said firstand second functions of temperature to be substantially matched over a range of temperature thereby resulting in a dynamic equilibrium of vaporized alkali atoms at substantially said optimum vapor pressure over said range of temperature,

said first means cooperating with said second means for preventing the absorbing means from losing its absorbing efficiency and for decelerating said atoms supplied from said reservoir to obtain within the envelope the desired atomic density.

2. A cell as claimed in claim 1, wherein said second means comprise a paraffin layer deposited on said walls inside said envelope.

3. A cell as claimed in claim 2, wherein said first means comprise a small-diameter orifice of predetermined diameter between said bulb and said envelope.

4. A cell as claimed in claim 2, wherein said first means comprise a capillary tube of predetermined diameter and length between said bulb and said envelope.

5. A cell as claimed in claim 2, wherein said first means comprise a porous wall of predetermined porosity and thickness between said bulb and said envelope.

6. A cell as claimed in claim 1, wherein said second means comprise a filler gas at a predetermined pressure inside said envelope.

7. A cell as claimed in claim 6, wherein said second means consist of the glass of saidwalls, and said first means further comprise a neck of predetermined diameter and length between said bulb and said envelope. 

1. An optical resonance cell adapted to substantially maintain an optimum alkali metal vapor pressure therein over a range of temperatures, said cell comprising; an envelope having transparent walls, a reservoir containing an alkali metal and connected to said envelope for supplying vaporized alkali atoms thereto, first means disposed between said reservoir and said envelope for limiting said supply of alkali atoms to a first rate which is a first predetermined function of temperature, and second means within said envelope for absorbing said vaporized alkali atoms from said envelope at a second rate which is a second predetermined function of temperature, said first and second means being constructed relative to each other and cooperating with each other to cause said first and second functions of temperature to be substantially matched over a range of temperature thereby resulting in a dynamic equilibrium of vaporized alkali atoms at substantially said optimum vapor pressure over said range of temperature, said first means cooperating with said second means for preventing the absorbing means from losing its absorbing efficiency and for decelerating said atoms supplied from said reservoir to obtain within the envelope the desired atomic density.
 2. A cell as claimed in claim 1, wherein said second means comprise a paraffin layer deposited on said walls inside said envelope.
 3. A cell as claimed in claim 2, wherein said first means comprise a small-diameter orifice of predetermined diameter between said bulb and said envelope.
 4. A cell as claimed in claim 2, wherein said first means comprise a capillary tube of predetermined diameter and length between said bulb and said envelope.
 5. A cell as claimed in claim 2, wherein said first means comprise a porous wall of predetermined porosity and thickness between said bulb and said envelope.
 6. A cell as claimed in claim 1, wherein said second means comprise a filler gas at a predetermined pressure inside said envelope.
 7. A cell as claimed in claim 6, wherein said second means consist of the glass of said walls, and said first means further comprise a neck of predetermined diameter and length between said bulb and said envelope. 